1. Field of the Invention
The present invention relates to a fundus blood vessel monitoring or observing apparatus for measuring a blood-flow velocity or speed in the blood vessel in an eye fundus.
2. Description of the Related Art
Hitherto, there has been known a laser doppler fundus blood-flow meter or velocimeter as an apparatus for tracking of a blood vessel in an eye fundus. In the case of the fundus blood-flow meter, it is important that a blood vessel, through which a blood flow having a blood flow velocity to be measured flows, is selected, that the tracking of the selected blood vessel is performed, and that the selected blood vessel is irradiated, with measurement beams during the measurement of the blood flow velocity. Thus, there has been proposed a system which is provided with a detection mechanism for detecting a nictitation (or blink) of an eye to be examined. This system is adapted so that the tracking of the blood vessel is temporarily interrupted when nictitation of the eye occurs during the measurement of the blood flow velocity, and so that the tracking of the blood vessel is resumed by calculating a restart position when nictitation of the eye is finished and the system is brought into a condition in which the tracking thereof is enable to be performed. Further, there has been known another fundus blood-flow meter adapted so that the tracking of a blood vessel ceases when detecting an abnormality in a value represented by a tracking-position signal.
However, the aforementioned conventional system that is adapted to interrupt the tracking of a blood vessel has the drawback that, when resuming the tracking thereof, it cannot determine whether or not the tracking is performed on the same blood vessel as before the interruption of the tracking. Further, the blood flow meter adapted to detect an abnormality in a tracking-position signal has the following defect. Namely, in the case where another (adjacent) blood vessel is present in the vicinity of a blood vessel on which the tracking is performed, according to the conditions of eyelashes, tears, and fixation (of an eye), no abnormality is detected in the tracking position signal, so that it is erroneously determined that the measurement of a blood-flow velocity is normally finished.